Tilting landing gear systems and methods

ABSTRACT

Systems and methods for mechanically rotating an aircraft about its center-of-gravity (C G ) are disclosed. The system can enable the rear, or main, landing gear to squat, while the nose landing gear raises to generate a positive angle of attack for the aircraft for takeoff or landing. The system can also enable the nose gear and main gear to return to a relatively level fuselage attitude for ground operations. The system can include one or more hydraulically linked hydraulic cylinders to control the overall height of the nose gear and the main gear. Because the hydraulic cylinders are linked, a change on the length of the nose cylinder generates a proportional, and opposite, change in the length of the main cylinder, and vice-versa. A method and control system for monitoring and controlling the relative positions of the nose gear and main gear is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure is a continuation of, and claims priority under35 U.S.C. § 120 to, U.S. patent application Ser. No. 15/198,611, filedJun. 30, 2016, of the same title, which is hereby incorporated byreference as if fully set forth below.

BACKGROUND

Conventional aircraft consist essentially of a wing section and afuselage. This so-called “tube and wing” configuration enablesconvenient packaging of passengers and cargo, but has certain drawbacks.In most cases, passengers are seated on a deck disposed approximately onthe vertical centerline of the fuselage, while cargo is stowed beneath.This enables a relatively wide, flat floor for seats and separates cargooperations from passenger loading and unloading. Passengers can beloaded via one or more passenger doors, while cargo can be loaded fromone or more cargo hatches on the underside or sides of the fuselage.This configuration also provides a relative constant fuselage crosssection (less the nose and tail cones), enabling a substantiallypercentage of the available volume of the fuselage to be utilized.

While convenient from a packaging standpoint, the tube and wingconfiguration is not particularly efficient. This is because thefuselage provides little or no lift, yet introduces substantial drag.Thus, the wing must provide substantially all of the lift required forthe aircraft to fly. This configuration requires a wing that is larger,thicker, and/or more cambered than would otherwise be required (i.e., ifthe fuselage provided a larger percentage of the required lift). Thisresults in a wing with higher lift, but proportionately higher drag.Thus, the engines must provide enough thrust to overcome the drag fromboth the fuselage and the (now higher drag) wing.

In a blended wing configuration, on the other hand, both the fuselageand the wing provide lift. As the name implies, the blended wing blendsthe wing and fuselage together to provide a single, lift-producing body.In this configuration, the fuselage serves to both carry passengersand/or cargo and to provide a significant portion of the lift. As aresult, the wing portion can be smaller for a given payload. Thus,blended wing aircraft tend to have significantly lower overall drag andcan carry larger payloads while consuming less fuel.

Due to their unconventional shape, however, blended wing aircraft canpresent some challenges with regard to packaging. In other words,because the shape of the fuselage is more irregular than a conventionaltube-shaped fuselage, providing storage for cargo, equipment,passengers, and other components can be challenging. In particular, asshown in FIGS. 1A and 1B, finding a suitable place to stow the retractedlanding gear 105 can be challenging. In general, it is desirable toplace the main, or rear, landing gear 105 a fairly close to the centerof gravity, C_(G), of the aircraft. This placement reduces theaerodynamic forces that must be generated by the flight control surfaces120 (e.g., elevons 110 and/or flaps 115) to rotate the aircraft ontake-off. In other words, if the main landing gear 105 a is placed toofar from the C_(G), the flight surfaces cannot overcome the weight ofthe aircraft acting on such a large lever arm, L_(MG), for the purposesof takeoff rotation.

As shown in FIG. 1A, therefore, from a weights and balances standpoint,it is desirable to place the main gear 105 a as close to the C_(G) aspossible. In addition, the maximum width, or track, of the landing gearis limited by regulation to ensure landing gear/runway compatibility. Ina blended wing design, however, this unfortunately places the landinggear in the middle of the desired passenger compartment (on a singlelevel aircraft) or in the middle of the cargo compartment (on amulti-level aircraft). This reduces seating and/or cargo capacity andmakes packaging, interior aesthetics, and utility more difficult, amongother things.

As shown in FIGS. 2A and 2B, one solution is to simply move the mainlanding gear 105 a rearward out of the passenger compartment 125.Unfortunately, this places the main landing gear 105 a at a substantialdistance from the C_(G). This, in turn, creates a large lever armL_(MG), between the C_(G) and the contact patch of the main landing gear105 a. In this configuration, the elevons 110 and/or flaps 115 arelikely unable to generate enough negative lift at the rear of the wingto rotate the plane for takeoff. Thus, one problem—clearing thepassenger and/or cargo compartment—has been traded foranother—increasing takeoff distance or not being able to take off atall. Of the two, taking off is clearly more important in an aircraft.

What is needed, therefore, is a system and method for rotating theaircraft for takeoff using something other than the aerodynamic controlsurfaces. After takeoff, the location of the main landing gear 105 a isrelevant only to the overall weights and balances of the plane (e.g.,center of lift, C_(L) vs C_(G)). The system should be simple and robustand provide pilots with a similar tactile experience as a conventionalconfiguration. It is to such systems and methods to which examples ofthe present disclosure are primarily directed.

SUMMARY

Examples of the present disclosure relate to a tilting, or rotating,landing gear system. The system enables an aircraft to be rotated aboutits center-of-gravity (C_(G)) regardless of the placement of the landinggear. In this configuration, the landing gear of the aircraft can beplaced farther from the C_(G) than would otherwise be possible. Thesystem provides a balanced hydraulic or mechanical system to reduce theeffort required to rotate the aircraft to assume the desiredangle-of-attack (AOA) for various procedures, such as takeoff, landing,ground operations, and other operational regimes. The system reduces theaerodynamic forces required by balancing mechanical and hydraulic forcesabout the C_(G) and can also include mechanical or hydraulic means inaddition to the aerodynamic forces provided by the aerodynamic surfacesof the aircraft (e.g., ailerons, elevons, and/or flaps).

In some examples, the system can be a passive hydraulic system withhydraulic, pneumatic, or hydro-pneumatic cylinders mounted to the noseand main gears. The cylinders can be hydraulically connected such thatan upward movement in a nose cylinder causes a proportional movement intwo or more main cylinders, and vice versa. The cylinders can bebalanced such that the hydraulic and mechanical forces (e.g., leverage)are balanced about the C_(G), such that very little aerodynamic force isrequired to rotate the aircraft for takeoff.

In some examples, to achieve a positive AOA for takeoff, for example, ahydraulic valve between the main cylinder and nose cylinder can beopened. The cylinders can be sized such that, when the valve is in theopen position with the aircraft on the ground, hydraulic fluid flowsfrom the main cylinder(s) to the nose cylinder(s) causing the nose gearto extend and the main gear to squat. The relative position of the maingear(s) and the nose gear(s) can be locked by closing the hydraulicvalve (i.e., when the desired AOA has been achieved). The system caninclude an AOA for landing, takeoff, ground operations, or maintenance,among other positions.

In flight, the valve can be opened and the weight of the main gear, forexample, can cause the fluid to move from the nose cylinder back to themain cylinder. This extends the main gear and lowers the nose gear to aground position, stowed position, or landing position, among otherpositions. During ground operations, the cylinders can be locked suchthat the aircraft is substantially level to the ground (i.e., theaircraft has a substantially zero AOA). In some examples, the system canalso include a hydraulic pump to move hydraulic fluid from the nosecylinder to the main cylinder, and vice-versa.

Examples of the present disclosure can also include a control system anda method for mechanically rotating the aircraft during variousprocedures, such as take-off or landing. The control system can lock theaircraft in a substantially level attitude when on the ground (e.g., ata substantially zero AOA). When the aircraft reaches the appropriatespeed and the pilot is pulling back on the control stick with at least aminimum force, the control system can open the hydraulic valve enablingthe main gear to squat and the nose gear to extend to the desired AOA.This could include, for example, a rotation speed, or V₁, for takeoff,for example, or a reference speed, V_(REF), for landing. The controlsystem can close the hydraulic valve to lock the landing gear at thedesired AOA. After takeoff, when the control system determines that thelanding gear is unloaded—and the aircraft is airborne—the control systemcan close the valve to lock the landing gear in the position necessaryfor strut retraction. After landing, when the aircraft has sufficientlyreduced its speed, the control system can close valve to lock the planein a substantially level, ground position.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B are plan and side views, respectively, depicting ablended-wing aircraft with the landing gear in a convention locationnear the center-of-gravity (C_(G)) and inside the passenger or cargocompartment.

FIGS. 2A and 2B are plan and side views, respectively, depicting ablended-wing aircraft with the main landing gear in a rearward locationfarther from the C_(G) to improve packaging.

FIGS. 3A and 3B are side views depicting a blended-wing aircraft withthe main landing gear in a rearward location with a tilting landing gearsystem in the level, or ground position (FIG. 3A) and in theangle-of-attack (AOA)—e.g., takeoff or landing—position (FIG. 3B), inaccordance with some examples of the present disclosure.

FIGS. 4A and 4B are side views depicting a blended-wing aircraft with adirect-hydraulic tilting landing gear system in the level, or groundposition (FIG. 4A) and in the AOA position (FIG. 4B), in accordance withsome examples of the present disclosure.

FIGS. 5A and 5B are side views depicting a blended-wing aircraft with alever-actuated tilting landing gear system in the level, or groundposition (FIG. 5A) and in the AOA position (FIG. 5B), in accordance withsome examples of the present disclosure.

FIG. 6A is a side view depicting a blended-wing aircraft with a rearwardpivoting swingarm landing gear system, in accordance with some examplesof the present disclosure.

FIG. 6B is a side view depicting a blended-wing aircraft with a forwardpivoting, folding swingarm landing gear system, in accordance with someexamples of the present disclosure.

FIG. 7A is a front view depicting a landing gear with direct-linearactuators, in accordance with some examples of the present disclosure.

FIG. 7B is a front view depicting a landing gear with linear actuatorsacting via a swingarm, in accordance with some examples of the presentdisclosure.

FIGS. 8A and 8B are front views depicting a telescoping landing gear inthe extended position (FIG. 8A) and the retracted, or collapsed,position (FIG. 8B), in accordance with some examples of the presentdisclosure.

FIG. 9 is a schematic diagram of a control system for the rotatinglanding gear system, in accordance with some examples of the presentdisclosure.

FIG. 10 is a flowchart depicting a method for passively controlling therotating landing gear system on takeoff, in accordance with someexamples of the present disclosure.

FIG. 11 is a flowchart depicting a method for actively controlling therotating landing gear system on takeoff, in accordance with someexamples of the present disclosure.

FIG. 12 is a flowchart depicting a method for controlling the rotatinglanding gear system on landing, in accordance with some examples of thepresent disclosure.

DETAILED DESCRIPTION

Examples of the present disclosure related generally to aircraft landinggear, and specifically aircraft landing gear that enables the aircraftto be tilted, or rotated, for takeoff or landing, for example, with verylittle force from the aircraft's aerodynamic control surfaces. In someexamples, the system can include one or more interconnected hydrauliccylinders that enable the aircraft landing struts to extend in the frontand collapse in the back to provide the desired takeoff or landingattitude. In some examples, the hydraulic cylinders can be sized andshaped to provide the desired first configuration on the ground and thenreturn to the default, second configuration after takeoff. In otherembodiments, the hydraulic cylinders can be linked to levers sized andshaped to provide the desired effect.

To simplify and clarify explanation, the disclosure is described hereinas a system and method for use with a blended wing aircraft. One skilledin the art will recognize, however, that the disclosure is not solimited. While the system is useful in conjunction with blended wingaircraft due to some unique packaging constraints, it should beunderstood that the system can just as easily be used for conventionaltube and wing, delta wing, and other aircraft configurations. The systemcan also be used as a safety measure to ensure takeoff or landingrotation in a plane that is, for example, malfunctioning orinadvertently misloaded. In addition, the system could also be used forground-based equipment, such as loaders, semi-trucks, and otherequipment that require tilting or rotation during use.

The manufacturing methods, materials, and systems described hereinafteras making up the various elements of the present disclosure are intendedto be illustrative and not restrictive. Many suitable materials, struts,systems, and configurations that would perform the same or a similarfunction as the systems described herein are intended to be embracedwithin the scope of the disclosure. Such other systems and methods notdescribed herein can include, but are not limited to, vehicles, systems,networks, materials, and technologies that are developed after the timeof the development of the disclosure.

As discussed above, it is often convenient to place the main landinggear toward the rear of the aircraft for packaging purposes. This tendsto move the landing gear behind the passenger and/or cargo compartmentsenabling more, or more convenient, passenger and cargo compartments.Unfortunately, this also tends to move the landing gear away from theC_(G) of the aircraft. This, in turn, increases the amount of forcerequired to rotate the aircraft for takeoff or landing. This rearwardlanding gear configuration may require five times the force or more, torotate the aircraft than can be produced by the aerodynamic surfaces ofthe wing at takeoff speeds. What is needed, therefore, is a system andmethod that assists, or replaces, the forces provided by the aerodynamicsurfaces with mechanical forces of sufficient force.

To this end, as shown in FIGS. 3A and 3B, examples of the presentdisclosure can comprise a system 300 comprising a main gear 305 that cansquat and/or a nose gear 310 that can extend to mechanically provide thedesired angle-of-attack (AOA or a) for takeoff and/or landing. In someexamples, the system 300 can comprise two or more main gears 305 a andone or more nose gear 310. In some examples, the system 300 can comprisestandard oleo struts 315 (e.g., air-oil pneumatic struts) mounted on oneor more actuators 320. In some examples, as discussed below, theactuators 320 can be cylinders that are hydraulically or pneumaticallylinked, such that when one hydraulic cylinder 320 collapses the otherhydraulic cylinder 320 extends, and vice-versa. In other examples, thehydraulic cylinder 320 can be independently controlled to work inconcert. In a preferred embodiment, the hydraulic cylinder 320 cancomprise hydraulic cylinders that are also hydraulically linked.

As shown in FIG. 3A, therefore, in the level, or ground, configuration,the aircraft can be substantially level. In this configuration, thehydraulic cylinders 320 can be positioned such that the oleo struts 315suspend the aircraft at a substantially level attitude with respect tothe ground. This can enable passengers and cargo to be loaded onto theaircraft in the conventional manner. This can also enable the aircraftto be taxied for takeoff without unnecessarily affecting the pilot'sview of the ground or adversely affecting ground handling. In otherexamples, the aircraft can have a slightly nose heavy configuration, forexample, such that when the aircraft is on the ground, the nosehydraulic cylinder 320 b is fully retracted and the main hydrauliccylinder 320 a is fully extended. As discussed below, in some examples,for safety purposes, the hydraulic cylinders 320 can be locked in thelevel position anytime the aircraft is on the ground and below apredetermined speed unless otherwise overridden—e.g., for maintenancepurposes.

As shown in FIG. 3B, however, to enable the aircraft to rotate fortakeoff or landing, the main hydraulic cylinder 320 a can collapse andthe nose hydraulic cylinder 320 b can extend to provide the desired AOA.In this configuration, as with conventional landing gear, the oleostruts 315 react to impacts and undulations on the ground, but thesemotions are measured in inches, quite small relative to the strokeneeded for the tilting system. As the hydraulic cylinders collapse andextend, however, the overall height of the strut/cylinder assembly 325changes.

Thus, as the main hydraulic cylinder(s) 320 a (i.e., two or more mainhydraulic cylinder 320 a for the two or more main gears 305 a) retracts,the rear strut/cylinder assembly 325 a squats. Conversely, as the nosehydraulic cylinder 320 b (i.e., the cylinder for the nose gear 310)extends, the nose strut/cylinder assembly 325 b extends. This has theeffect of lowering the rear of the aircraft and raising the front of theaircraft to simulate takeoff rotation and/or landing flare.

Notably, however, this attitude is achieved with the landing gear 305,310 still on the ground. In addition, as discussed below, the locationand size of the hydraulic cylinders 320 can be such that they areessentially in equilibrium about the C_(G). In this manner, the system300 can rotate the aircraft with very little force provided by theaerodynamic surfaces of the wing. This (1) overcomes the aforementionedissues related to overcoming a large L_(MG) and (2) does so with thewing in a more aerodynamically efficient configuration. Because rotationrequires much less negative lift and thus, deflection of the elevons 110(or elevons in a tailless configuration) and/or flaps 115, the wing isalso in a “cleaner” aerodynamic configuration (at least initially). Inother words, significantly less negative lift is required at the back ofthe wing to generate the rotation moment, enabling the wing to providegreater positive lift for takeoff. This, in turn, can reduce takeoffspeed, and therefore takeoff distance.

Upon takeoff, once the main gear 305 has cleared the tarmac, thelocation of the main gear 305 is no longer relevant from an aerodynamicstandpoint. Once aloft, the location of the main gear 305 is relevantonly from a weights and balances standpoint, which can be accounted forwith fuel, cargo, and/or passenger weight, among other things. At orbefore liftoff, therefore, the flight control surfaces 120 can bepositioned to provide the necessary aerodynamic forces to maintain thedesired AOA for climb out.

Of course, while shown and described with hydraulic cylinders 320,pneumatic cylinders and other types of linear or rotary actuators couldbe used. The system 300 could utilize linear actuators, for example,electrically driven by the aircraft's electrical system. The system 300could also utilize servo motors, for example, with a rack and pinion orpushrod actuation to the landing gear 305, 310. Indeed, rather thanusing separate hydraulic cylinders 320, as shown, the system 300 coulduse lengthened versions of the existing oleo struts 315 interconnectedin a similar manner. This configuration might reduce weight andcomplexity if sufficient space is available in the airplane for thelengthy struts 315 and the volume swept by the rotation angle needed forretraction. Thus, any type of mechanism that can enable the main gear305 to squat and/or the nose gear 310 to lift can provide the necessaryAOA.

As shown in FIGS. 4A and 4B, in some examples, the system 400 can beessentially passive. In this configuration, the hydraulic cylinders 320can be linked and can be sized and shaped such that they are essentiallyhydraulically neutral about the C_(G). In other words, the total area ofthe piston(s) for the nose hydraulic cylinder 320 b and the total areaof the piston(s) for the main hydraulic cylinder 320 a combined withtheir relative distances from the C_(G) can be calculated to balance theaircraft about the C_(G).

As shown, in some examples, the main hydraulic cylinder 320 a can have alarger total piston surface area, A_(MG), (i.e., the combined, or total,piston surface area of the main hydraulic cylinders 320 a, if there aremultiple main gears 305 a) than the total piston surface area, A_(NG),of the nose hydraulic cylinder 320 b. In the configuration, thedistance, L_(NG), from the nose gear 310 to the C_(G) can be larger thanthe distance, L_(MG), from the main gear 305 to the C_(G) to producehydro-mechanical equilibrium. Thus, in Equation 1:

$A_{NG} = \frac{A_{MG}}{\frac{L_{NG}}{L_{MG}}}$

So, for example, if L_(NG)=3×L_(MG), then the A_(NG)=⅓ A_(MG) (from 1,A_(NG)=A_(MG)/3). The hydraulic cylinders 320 can also be linked with asuitably sized hydraulic pipe 405 (e.g., a pipe or hose).

This system allows very small force to rotate the aircraft about itsC_(G) despite the placement of the landing gear 305, 310 farther fromthe C_(G). In other words, by balancing the hydraulic forces between thenose hydraulic cylinder(s) 320 b and the main hydraulic cylinder(s) 320a, a virtual pivot about the C_(G) is created. Thus, a small downwardforce at the rear of the wing from the flight control surfaces 120 cancause the aircraft to rotate for takeoff. Similarly, a small brakingforce from the aircraft's brakes can cause the aircraft to de-rotatefrom the landing position to the ground position, for example.

In some examples, it may be desirable to include a hydraulic valve 410between the hydraulic cylinders 320. In this manner, the hydrauliccylinders 320 can be locked in a particular position. The hydraulicvalve 410 can be, for example, a ball valve, gate valve, or throttlevalve.

In the level, or ground position, therefore, both hydraulic cylinders320 can be positioned such that the oleo struts 315 are in substantiallythe same position and the aircraft fuselage is substantially level. Insome examples, the aircraft may have a very slightly nose heavyconfiguration. This can be achieved passively with the difference indeadweight of the landing gear, for example, or by using a small pump435 to provide a slight bias of fluid to the main hydraulic cylinder 320a. When the hydraulic valve 410 is open (or there is no hydraulicvalve), therefore, the nose hydraulic cylinder 320 b can retract and themain hydraulic cylinder 320 a can extend. In some examples, the aircraftcan be in the ground position when the nose hydraulic cylinder 320 b iscompletely retracted, or “bottomed out.”

As shown in FIG. 4B, to achieve the desired AOA, the nose hydrauliccylinder 320 b can be extended and the main hydraulic cylinder 320 a canbe retracted. This can be achieved in a number of ways. Since the system400 is balanced about the C_(G), for example, a small downward forcefrom the flight control surfaces 120 can cause the aircraft to rotateabout the C_(G) to the AOA position.

If a hydraulic valve 410 is included, the hydraulic valve 410 can firstbe placed in the open position. Because the aircraft is in equilibrium,however, opening the hydraulic valve 410 does not, in itself, create anyrotation. As before, the rotation can be provided by small forcesprovided by the flight control surfaces 120. In some examples, theaforementioned pump 435 can be reversed to cause fluid to flow from themain hydraulic cylinder 320 a to the nose hydraulic cylinder 320 b.This, in turn, causes the main gear 305 to squat and the nose landinggear 130 to extend creating the desired AOA. In any configuration (i.e.,with or without a hydraulic valve 410 or a pump 435), the amount ofenergy required to cause the rotation is significantly smaller becausethe aircraft is essentially balanced about the C_(G).

As discussed below, in some examples, the system 400 can include acontrol system to control the position of the hydraulic cylinders. Insome examples, the system 400 can comprise one or more sensors to detectthe AOA. In some examples, the system 400 can include a tilt sensor 420disposed on the aircraft to detect the AOA. In some examples, the tiltsensor 420 can comprise the attitude sensor included in the aircraft'sexiting avionics package. In other examples, the tilt sensor 420 cancomprise a separate gyro, accelerometer, or similar sensor to detect theAOA. In other examples, the system 400 can include one or more positionsensors 425 located on the hydraulic cylinders 320. Based on theposition of the hydraulic cylinders 320 and the geometry of the system,the AOA can be calculated.

In still other examples, the system 400 can include one or more switches430 located on the hydraulic cylinders 320. In this configuration, thesystem 400 can simply comprise two or more positions for various flightsituations. The system 400 can include one switch 430 a for eachcylinder in the ground position (FIG. 4A), for example, and one switch430 b on each cylinder for the takeoff position (FIG. 4B). The system400 can also comprise additional switches for additional positions(e.g., landing, heavy payload, etc.).

In some examples, the system 400 can be completely passive. In otherwords, in some examples, the system 400 can move from the groundposition (FIG. 4A) to the takeoff position (FIG. 4B) based solely on thesmall aerodynamic force provided by the flight control surfaces 120.Similarly, after takeoff, the system 400 can move back to the groundposition, for example, because the total weight of the main gear 305 isgenerally significantly heavier than the total weight of the nose gear310. After takeoff, therefore, the hydraulic valve 410 can remain open(or be reopened) to enable the total weight of the main gear 305 toextend the main hydraulic cylinder 320 a and compress the nose hydrauliccylinder 320 b back to the level position. In other examples, asdiscussed below, hydraulic pumps, motors, or other power assist can beused.

As mentioned, the system 400 can also comprise a hydraulic motor or pump435. The pump 435 can be used to actively reposition the landing gear305, 310 despite loading. In other words, in some examples, it may bedesirable to raise the nose gear 310 despite the fact that the aircraftis in a nose heavy configuration. In this configuration, opening thehydraulic valve 410 may cause the nose hydraulic cylinder 320 b tocollapse. In this case, the pump 435 can be activated to provide thedesired forward pressure. In some examples, the pump 435 can bereversible, enabling it to pump fluid in either direction to affectrotation in either direction (e.g., nose up/mains down and nosedown/mains up). In some examples, the pump 435 can be activated onlywhen the system 400 determines that the landing gear 305, 310 is notmoving in the desired direction.

In addition to providing equilibrium about the C_(G), the differentrelative sizing of the hydraulic cylinders 320 also cause aproportionally different stroke, S, for each of the cylinders. Thisrelationship is given in Equation 2:

$\frac{L_{NG}}{L_{MG}} = \frac{S_{NG}}{S_{MG}}$In other examples, as shown in FIGS. 5A and 5B, the hydraulic cylinders505 can be augmented with levers 510 to provide the desired effect. Inother words, in some examples, as shown in FIGS. 5A and 5B, the levers510 can be used to increase the travel of the nose gear 310 or main gear305 to enable a smaller (shorter) hydraulic cylinder 505 to be used.Thus, while the main gear 305 may be substantially in line with the rearhydraulic cylinder 505 a, the nose gear 310 may be offset from the fronthydraulic cylinder 505 b with a nose gear lever 510 b. In this manner, arelatively small movement of the front hydraulic cylinder 505 b resultsin a larger movement of the nose gear 310. Thus, a hydraulic cylinder505 with a shorter stroke can be used, if desired, for packaging,weight, or other reasons.

In some examples, the mechanical advantage/disadvantage of the levers510 can also be used to adjust the equilibrium between the fronthydraulic cylinder 505 b and rear hydraulic cylinder(s) 505 a. As shownin FIG. 5A, the rear hydraulic cylinder 505 a acts substantiallydirectly on the main gear 305, while the front hydraulic cylinder 505 bacts on the nose gear 310 via the nose gear lever 510 b at a distanceL_(NG). In this configuration, the nose gear 310 travels farther thanthe main gear 305 for a given hydraulic cylinder 505 stroke, but also isacted upon with less force. This can serve to distribute the forcesappropriately based on the fact that the nose gear 310 generally carriesless weight than the main gear 305.

This can also enable the same size hydraulic cylinder 505 to be used forboth the nose gear 310 and the main gear 305. Because the rear hydrauliccylinder 505 a acts substantially directly on the main gear 305, whilethe front hydraulic cylinder 505 b acts via a lever arm,L_(NG-MOUNT)/L_(PISTON-MOUNT), the same size hydraulic cylinder 505 canbe used to provide hydraulic equilibrium. In other words, the mechanicaladvantage provided by the distance from the nose gear to the C_(G),L_(NG), is offset by the mechanical disadvantage of the nose gear lever510 b. This relationship is given in Equation 3:

A_(NG) = A_(MG)$\frac{L_{{NG}\text{-}{MOUNT}}}{L_{PISTONMOUNT}} = {\frac{L_{NG}}{L_{MG}} = \frac{S_{NG}}{S_{MG}}}$

Thus, if we once again assume that L_(NG)=3×L_(MG), but that thehydraulic cylinders 505 have the same total area, thenL_(NG-MOUNT)/L_(PISTON-MOUNT)=3 provides hydro-mechanical equilibrium.This approach allows all of the tilting hydraulic cylinders to beidentical for lower cost.

Of course, the linkages for the landing gear can take many forms. Asshown in FIG. 6A, for example, the nose gear 310 can be mounted on arear-swinging swingarm 605. As discussed above, this can enable the nosehydraulic cylinder 320 b to have a shorter stroke, while providingsufficient travel for the nose gear 310. As shown, the nose gear 310 canhave at least three positions. In the first position, {circle around(1)}, the rear-swinging swingarm 605 is in the fully retracted position.In this position, the nose gear 310 is fully retracted inside thefuselage to enable the landing gear doors to be closed, for example. Inthe second, or intermediate position, {circle around (2)}, therear-swinging swingarm 605 is in substantially the same position, butthe nose gear 310 is rotated to a position that supports the fuselagesuch that it is substantially level with the ground. This can bereferred to as the “ground position” in that it can enable the aircraftto taxi and to be loaded and unloaded in the convention manner.

In the third position, {circle around (3)}, or AOA position, therear-swinging swingarm 605 (and thus, the nose gear 310) can be loweredto raise the nose of the aircraft to create the desired AOA. Asdiscussed above, the AOA position can be used to place the aircraft inthe proper configuration for takeoff, for example, while requiring verylittle force to be supplied by the aerodynamic surfaces (e.g., onlyenough to upset the equilibrium). In this manner, the aircraft can berotated for takeoff, for example, despite there being a large distance(and a resulting large moment) between the landing gear and the C_(G).

In some examples, the high AOA position can also be used for landing.For most aircraft, lowering the nose as quickly as possible reduceslanding distance. However, for some aircraft holding the nose at highAOA adds drag to reduce landing distance. In this instance, the nosegear 310 can be placed in the high AOA position prior to landing suchthat, when the nose gear 310 touches down, the aircraft maintains thehigh angle for deceleration. The aircraft can then be slowly rotated tothe ground position during deceleration. In some examples, the forcerequired to cause the (de)rotation on landing can be provided by theaircraft's braking system.

In some examples, the hydraulic pipe 405 can be sized and/or can includean orifice, to provide the desired rotation rate. In other words, thehydraulic pipe 405 can include a restriction to prevent the nose gear310 from collapsing in an uncontrolled manner. In some examples, thesystem 500 can include a pump 435 to provide a slight forward bias. Inthe manner, the nose gear 310 can maintain the AOA position untilovercome by braking forces. At this point, the pump 435 can be turnedoff or reversed to enable the system 500 to rotate back to the groundposition.

In some examples, the AOA position for landing and takeoff can place thefuselage at the same AOA. In other examples, the AOA position cancomprise at least two different positions—a landing AOA position and atakeoff AOA position.

As shown in FIG. 6B, in some examples, the system can include aforward-swinging swingarm 610. Similar to the rear-swinging swingarm605, the forward-swinging swingarm 610 can enable a shorter stroke nosehydraulic cylinder 320 b to be used. In addition, in some examples, theforward-swinging swingarm 610 can include an additional pivot 615 toenable the oleo strut 315 to be folded when in the first position,{circle around (1)}. In this manner, the nose gear 310 can be stowed ina smaller space when in the first position. As before, theforward-swinging swingarm 610 can also comprise a ground position,{circle around (2)}, for taxiing and ground operations, and an AOAposition, {circle around (3)}, for takeoff and/or landing.

As before, regardless of the nose gear 310 configuration, in addition toraising the nose, the main gear 305 can also squat to provide additionalrotation by simply collapsing the main hydraulic cylinder 320 a. In theground position, on the other hand, the main hydraulic cylinder 320 acan be raised to place the aircraft in a substantially level positionwith respect to the ground.

As shown in FIG. 7A, the nose gear 310 or main gear 305 can be mountedto the hydraulic cylinders 320 in a substantially linear manner. Inother words, in some examples, the oleo strut 315 can be mounteddirectly to the hydraulic cylinders 320 using a brace 705, for example,or other suitable means. In this configuration, extension or retractionof the hydraulic cylinders 320 results in the same extension orretraction of the landing gear 305, 310. This can enable the system tobe relatively simple, compact, lightweight, and robust.

In other examples, as shown in FIG. 7B, the nose gear 310 or main gear305 can be mounted to the hydraulic cylinders 320 via the forward 610 orrearward 605 pivoting swingarm. In other words, in some examples, theoleo strut 315 for either landing gear 305, 310 can be mounted to thehydraulic cylinders 320 via the forward 610 or rearward 605 pivotingswingarm using a brace 705, for example, or other suitable means. Inthis configuration, extension or retraction of the hydraulic cylinders320 can create a proportionately larger extension or retraction of thelanding gear 305, 310. This can enable the hydraulic cylinders 320 to beshorter for a given landing gear 305, 310 travel, which can improvepackaging. In addition, as discussed above, the swingarm may includeadditional pivots to enable the landing gear 305, 310 to fold whenretracted. This can enable the landing gear 305, 310 to be stowed morecompactly when retracted in flight.

In still other examples, as shown in FIGS. 8A and 8B, the system caninclude a telescoping strut mount 805. In this configuration, the oleostrut 315 and landing gear 305, 310 can be mounted to the telescopingstrut mount 805. The telescoping strut mount 805, in turn, can be movedbetween a first, extended position (FIG. 8A) and a second, retractedposition (FIG. 8B). Thus, the telescoping strut mount 805 can place theaircraft in the AOA position at or near the first position and can placethe aircraft in the ground position at or near the second position. Insome examples, the ground position can be in between the first positionand the second position. In this configuration, the first, fullyretracted position may enable the nose gear 310 to squat for service,loading, or other reasons (i.e., the second position may be below theground position).

Examples of the present disclosure can also comprise a system 900 formonitoring and controlling the position of the landing gear. The system900 can include a controller 905 for receiving inputs and providingoutputs to control the position of the landing gear. The controller 905can comprise, for example, a dedicated microcontroller, a laptop ordesktop computer, a module, an integrated circuit, or other suitableelectronic device. The controller 905 can include a processor, one ormore types of memory, and one or more communication buses for connectionto aircraft systems, sensors, and or actuators.

The system 900 can also include one or more sensors to provideinformation to the controller 905. The system 900 can include, forexample, a ground speed indicator 910, and airspeed indicator 915, and astick force sensor 920. The system 900 can also include a hydraulicvalve position sensor 925, a hydraulic pressure sensor 930, a nose gearposition sensor 935, a main gear position sensor 940, and a strut loadsensor 945.

As the names imply, the ground speed indicator 910 and air speedindicator 915 provide the velocity of the aircraft with respect to theground and the air, respectively. In some examples, the ground speedindicator 910 can comprise a mechanical or electronic speedometermounted to the landing gear 305, 310. In other examples, the groundspeed indicator 910 can be provided by GPS, LORAN, or other suitablemeans. The air speed indicator 915 can comprise a pitot tube, or othersuitable pressure measurement device. In some examples, the system 900can use a standalone ground speed indicator 910 and air speed indicator915. In other examples, this information can be provided by existingavionics already installed on the aircraft.

In order to prevent the landing gear 305, 310 from rotating atinopportune times, the system 900 can prevent rotation until certainconditions are met. One possible condition is that the aircraft is at,or near, the proper rotation speed, or V₁, for example, or a proper, orreference, landing speed, V_(REF). A second possible condition is thatthe pilot is requesting the rotation. To this end, the system 900 canalso include a stick force sensor 920. As the name implies, the stickforce sensor 920 can measure the input of the pilot on the flightcontrol stick or yoke. Thus, when the force applied by the pilotrearward on the stick (“pulling up” in the stick) reaches apredetermined force, the controller 905 can initiate the landing gearrotation.

In some examples, such as in a system with active hydraulics (discussedabove), the system 900 can also include a hydraulic pressure sensor 930.This can ensure that the landing gear 305, 310 remains locked unless anduntil there is sufficient pressure to affect the desired rotation. Ofcourse, in a passive system, the hydraulic pressure sensor 930 may serveonly to ensure there is not a pressure leak, or other failure.

The system 900 can also include one or more nose gear position sensors935 and main gear position sensors 940. The position sensors 935, 940can determine that the landing gear 305, 310 is initially in the groundposition, for example. The position sensors 935, 940 can also determinewhen the landing gear 305, 310 is in the AOA position for takeoff and/orlanding. As discussed below, the controller 905 can close the hydraulicvalve to lock the landing gear 305, 310 in a particular position, basedon feedback from the position sensors 935, 940. The positions sensors935, 940 can comprise, for example, capacitive displacement sensors,piezo-electric transducers, potentiometers, proximity sensors (e.g.,optical), or rotary encoders (e.g., angular). The position sensors 935,940 can be linear and can be mounted directly on the hydraulic cylinders320, for example, or can be rotary and can be connected to the hydrauliccylinders 320 or landing gear 305, 310 via pivoting arms.

The system 900 can also include a strut load sensor 945. The strut loadsensor 945 can measure the load, or position, of the oleo strut 315.Thus, on takeoff or landing, the landing gear 305, 310 can be locked inposition based on whether the oleo strut 315 is loaded (on the ground)or not. On takeoff, the landing gear 305, 310 can be locked in the AOAposition, for example, until the strut load sensor 945 determines thatthe nose gear 310, main gear 305, or both has been unloaded indicatingthe aircraft is airborne. At this point, the system 900 can open thehydraulic valve 410, for example, to return the landing gear 305, 310 tothe desired position (e.g., the ground position). The controller 905 canalso enable the landing gear 305, 310 to be folded to the stowed,in-flight position.

The controller 905 can also provide outputs to control the system 900.The controller 905 can send the appropriate signal to the hydraulicvalve 410, for example, to open 950 or close 955 base on the inputs fromthe various sensors. If the controller 905 determines that (1) theground speed indicator 910 indicates the aircraft is at or near V₁ and(2) the stick force sensor 920 indicates that the pilot is applying apredetermined amount of rearward force on the stick (e.g., 5-10 lbs.),then the controller 905 can send a signal 950 to open the hydraulicvalve 410.

The hydraulic valve 410 enables hydraulic fluid to flow from the mainhydraulic cylinder 320 a to the nose hydraulic cylinder 320 b. Thisenables the main gear 305 to squat and the nose gear 310 to raise toachieve the AOA position.

When the strut load sensor 945 indicates that the nose gear 310, themain gear 305, or both have left the tarmac, the controller 905 can senda signal 950 to reposition the landing gear 305, 310 to the appropriateposition, after which the hydraulic valve 410 can be closed locking thegear in the proper position for retraction. As discussed above, in someexamples, the weight of the main gear 305 hanging down can enable themain gear 305 to re-extend from the AOA position and the nose gear 310to retract.

In some examples, a hydraulic motor or pump 435 can be used tofacilitate or expedite movement of hydraulic fluid between the hydrauliccylinders 320. In this configuration, in addition to sending a signal toopen 950 and close 955 the hydraulic valve 410, the controller 905 canalso send a signal to start 960 and stop 965 the pump 435. The pump 435can enable the hydraulic cylinders 320 to be repositioned regardless ofconditions such as, for example, weight destruction, aircraft attitude,and strut loading.

Examples of the present disclosure can also include a methods 1000, 1100for mechanically rotating an aircraft for various procedures (e.g.,takeoff, taxiing, and/or landing). In some examples, as mentioned above,the system 500 can be substantially passive, relying on weights andbalances combined with mechanical and/or hydraulic forces to essentially“balance” the aircraft on the landing gear. In other configurations, thesystem 400 can use pumps 435 and/or hydraulic valves 410 to activelymove and control the aircraft.

In the “passive” configuration, the method 1000 can rely on themechanical and hydraulic layout to effect movement of the aircraft. At1005, as in a convention aircraft, takeoff can begin with the enginesbeing throttled up to the takeoff power setting. In a conventionalaircraft, the pilot can simply pull back on the stick during the takeoffroll. When the aircraft reaches a predetermined velocity, or V₁, thevelocity of the plane will be such that the aerodynamic surfaces of theaircraft rotate the nose of the aircraft into the air about the maingear.

As discussed above, in a blended wing configuration with a rearward maingear 305 placement, for example, the flight control surfaces 120 wouldnormally be unable to create sufficient force to affect rotation. Thisis due in part to the longer distance between the main gear 305 and theC_(G) (e.g., as shown in FIGS. 2A and 2B). As discussed above, due tothe hydraulic equilibrium created by the size, shape, and positioning ofthe hydraulic cylinders 320, however, rotation about the C_(G) can beprovided with very little force from the flight control surfaces 120.

To this end, as the aircraft accelerates, at 1010 the pilot can applyrearward pressure on the stick to deflect the elevons 110 (or elevons,as the case may be). At 1015, when the aircraft reaches the speed atwhich the aerodynamic forces overcome the inertial of the aircraft, theaircraft can rotate about the C_(G) to the AOA for takeoff, AOA_(TO). Asdiscussed above, because the aircraft is essentially in equilibrium onthe ground, very little force is required for the aircraft to rotate.

As a result, the “Minimum Unstick Speed” (VMU) can be low enough that itis not the critical condition for establishing Takeoff Decision Speed(V₁). As a result, V1 will generally be a substantially lower speed thanwould be required with conventional landing gear for any aircraftconfiguration (e.g., tube and wing vs. blended wing). Indeed, in theblended wing configuration, the force can be reduced from a level thatcannot practically be generated using aerodynamics to a force that islower than is currently required in a conventional tube and wingconfiguration. This can also significantly improve takeoff distances andclimb out because the negative lift that the flight control surfaces 120create—that the wing must counteract for liftoff—is substantiallyreduced.

At 1020, when the aircraft reaches the minimum unstick speed, or V_(MU),the aircraft will takeoff. As mentioned above, because very little forceis required to rotate the aircraft, the amount of lift required toovercome the negative lift created by the elevons 110 and lift theaircraft is reduced. As a result, V_(MU), takeoff roll, and fuelconsumption, among other things, can be reduced.

At 1025, the landing gear 305, 310 can move to another position, such asthe ground position. In some examples, the main gear 305 can be heavierthan the nose gear 310. When the aircraft takes off, therefore, theweight of the main gear 305 can cause the main hydraulic cylinder 320 ato extend and the nose hydraulic cylinder 320 b to retract. In someexamples, the ground position can occur when the main gear 305 extendscompletely (i.e., to the “stops”) and the nose gear 310 retractscompletely. In this configuration, the landing gear 305, 310 naturallyand passively returns to the ground position in the air. Of course, thelanding gear 305, 310 could also be configured to return to a landingAOA position or a stowed position (e.g., the position in which thelanding gear 305, 310 takes up the minimum amount of space whenretracted).

At 1030, regardless of the position the landing gear 305, 310 returns to(e.g., ground position, landing AOA position, stowed position, etc.),once the landing gear 305, 310 has reached the desired position, thelanding gear 305, 310 can be retracted for flight. In the passiveconfiguration, no valves or pumps are required for takeoff and allaircraft and landing gear 305, 310 positioning is provided either byaerodynamic forces or by the relative weights of the landing gear 305,310. This passive system reduces the complexity of the system, which canreduce weight, cost, and maintenance, among other things.

In some examples, a more active approach may be desired. In the “active”configuration, therefore, at 1105, takeoff can once again begin with theengines being throttled up to the takeoff power setting. At 1110, thesystem can then monitor the ground speed until the velocity of theaircraft, V, is at or near the velocity at which the aircraft wouldnormally rotate for takeoff, V_(R) (e.g., V_(R) is approximately V₁ —10knots). Once this velocity is attained, at 1115, the system can thendetermine if the pilot is pulling back on the stick with at least aminimum stick force, effectively requesting rotation of the aircraftinto the AOA_(TO) position. At 1120, if both conditions are met—i.e.,V≥V₁ and F_(STICK)≥F_(MIN)—then the system can open the hydraulic valve410 to enable the aircraft to rotate to AOA_(TO).

At 1125, the system can determine if the aircraft has achieved thedesired AOA. The AOA may vary based on, for example, whether theaircraft is landing or taking off, temperature, humidity, and takeoffweight, among other things. As discussed above, the AOA can be measuredusing an accelerometer, gyro, or the onboard flight systems.

In some examples, as discussed above, the AOA can be a function ofsystem geometry. In this configuration, the system 300, 400 can forgothe hydraulic valve 410, or the hydraulic valve 410 can remain in theopen position. In other examples, it may be desirable to hydraulicallylock the aircraft at a particular AOA based on takeoff weight, weatherconditions, etc. In this configuration, at 1130, if α=α_(TO), the systemcan close the hydraulic valve 410 to lock the landing gear 305, 310 atAOA_(TO).

At 1130, the system can determine if the aircraft has lifted off. Asdiscussed above, this can be achieved by measuring the position of, orthe load on, the landing gear 305, 310. If the landing gear 305, 310 isfully “stroked out,” for example, then the system can determine that theplane has taken off. This can also be determined when the load on thelanding gear 305, 310, F_(LG), drops below a predetermined levelF_(MIN)—i.e., F_(LG)≤F_(MIN). This can also be determined by input froman altimeter, GPS, or other flight instrument on the aircraft.

Regardless of how liftoff is determined, at 1140, the hydraulic valve410 is either already open, or the method 1100 can open the hydraulicvalve 410 to enable the landing gear 305, 310 to reposition. In someexamples, as discussed above, the weight of the main gear can be used tosimply “pull” the fluid back from the nose hydraulic cylinder 320 b intothe main hydraulic cylinder(s) 320 a to retract the nose gear 310 andextend the main gear 305. In some examples, the valve can be left openuntil the landing gear 305, 310 returns to the ground position, forexample, or the AOA position for landing.

In other examples, the landing gear 305, 310 can be moved to a “stowedposition,” where the main gear 305 and nose gear 310 are moved to aposition in which the main gear 305 and/or nose gear 310 have a reducedvolume over the volume the landing gear 305, 310 occupies when deployed.In other words, a position that minimizes, or at least reduces, thestowed volume of the landing gear 305, 310. In other examples, thestowed position may also enable the main gear 305 or the nose gear 310to avoid an internal structure, for example. This can be useful, forexample, to enable the landing gear 305, 310 to be stowed in theavailable space, avoiding bulkheads or other equipment. Thus, thelanding gear 305, 310 may not necessarily be stowed in the minimum spaceavailable, for example, due to size and shape requirements.

At 1145, the system can determine that the landing gear 305, 310 is inthe correct position, which may be, for example, the landing, stowed, orground position. If the ground position is desired, then the system candetermine that the position of the landing gear 305, 310, or P_(LG), isequal to the ground position, or P_(GROUND), as discussed above. If thelanding gear 305, 310 (or rather, the hydraulic cylinders 320) is in thecorrect position, at 1150, the system can close the hydraulic valve 410to lock the gear in place.

Examples of the present disclosure can also comprise a method 1200 forrotating the aircraft upon landing. At 1205, the system can determine ifthe aircraft is at some appropriate speed to extend the landing gear.This may be a speed, V that is lower than the maximum flap extendedspeed, V_(LE), for example, but above the reference landing speed,V_(REF). At 1210, if the plane is at an appropriate speed, the gear canbe extended. At 1215, as before, the system can determine if the pilotis requesting a positive angle of attack. In some examples, the systemcan determine if the pilot is applying some positive backward pressureon the stick, or if F_(STICK)≥F_(MIN).

At 1220, if the pilot is pulling back on the stick, the system canactivate a pump 435 (e.g., a small turbo-pump) and/or open the hydraulicvalve 410. The pump 435 can be used to overcome any bias in the systemfor the main gear 305 to droop out when in the air. Thus, the pump 435can provide a slight rearward pressure to the system to cause the maingear 305 to pivot downward and the nose gear 310 to pivot upward. Asmentioned above, the hydraulic valve 410 may be used to lock theposition of the landing gear 305, 310, but is not required.

At 1225, the system can determine if the landing gear 305, 310 is in thecorrect position to provide the landing AOA, α_(L). As mentioned above,α_(L) can be the same as, or different than α_(TO). In some examples,α_(L) may be inherent in the landing gear 305, 310. In other words, whenthe main gear 305 is fully retracted and the nose gear 310 is fullyextended, then the aircraft is at the proper attitude for α_(L). Inother examples, the system can determine whether the landing gear 305,310 is in the correct position based on position sensors, or othermeans, and either idle the pump 435 or deactivate the pump 435. At 1230,the system can optionally close the hydraulic valve 410 to lock thelanding gear 305, 310 in the α_(L) position.

At 1235, the system can determine if the aircraft has touched down.Thus, if the force, F_(LG), on the landing gear 305, 310 is greater thansome minimum force, F_(MIN), the system can determine that the aircrafthas touched down. In some examples, this can be determined using somedirect method, such as measuring the position of, or pressure in, theoleo strut 315 or hydraulic cylinders 320. In other examples, the systemcan use input from an altimeter, instrument landing system (ILS), orother means to determine the aircraft is on the ground.

At 1240, after determining that the aircraft is on the ground, thesystem can deactivate or idle the pump 435. If the hydraulic valve 410was closed previously, the system can also open the hydraulic valve 410.In some examples, the weight transfer caused by the aircraft brakingupon landing can cause the nose gear 310 to pivot upward and the maingear 305 to pivot downward. In some examples, the aircraft may also havea slight forward weight bias to cause the aircraft to rotate slowly backto the ground position. In some examples, the pump 435 can be reversible(e.g., a small turbo-pump) to pump fluid from the nose gear 310 to themain gear 305. In some examples, the hydraulic circuit (e.g., thehydraulic pipe 405) can include an orifice, or other restriction, tocontrol the rate at which the nose gear 310 retracts and the main gear305 extends.

At 1245, the system can determine if the aircraft is in the groundposition, P_(GROUND). As before, this can be done in a variety of ways.In some examples, the aircraft is in P_(GROUND) when the nose gear 310is fully pivoted upward and the main gear 305 is fully pivoted downward.Thus, a slight weight bias towards the nose gear 310 can cause theaircraft to naturally assume P_(GROUND). At 1250, the hydraulic valve410 can optionally be moved to the closed position to hydraulically lockthe aircraft in P_(GROUND). This can be useful if the aircraft does notnaturally maintain P_(GROUND)—e.g., both the nose gear 310 and main gear305 are in some intermediate position in P_(GROUND). In some examples,it may simply be desirable to lock the position of the aircraft with thehydraulic valve 410 to prevent pitching during, for example, taxiing,refueling, loading, unloading, and maintenance.

Example 1

As discussed above, it is convenient to locate the main gear 305 behindthe rear-spar where there is ample room to store the retracted main gear305 in the “beaver-tail.” To allow the main gear 305 to be tens of feetbehind the C_(G) requires a new feature that works in concert with thenose gear 310. The location of the main gear 305 would normally preventrotation because there isn't enough aerodynamic control moment to liftthe plane's weight so far ahead of the main gear 305 axles.

To solve this, the main gear 305 and nose gear 310 are mounted tohydraulic cylinders 320 of approximately equal diameter for all threelanding gears (one nose gear 310 and two main gear 305). The nose gear310 and main gear 305 are plumbed together with a smaller hydraulic pipe405 and share hydraulic fluid. The system can be passive so pumps 435are not necessary, but may nonetheless be used. A hydraulic valve 410,between the nose gear 310 and main gear 305, can hydraulically lock thesystem when desired. Connecting the nose and main gear hydraulicallyallows the plane to rotate about the C_(G) with no jacking of theplane's weight. As the plane rotates nose-up, the fluid in the main gear305 cylinders is forced forward to fill the nose gear 310 cylinder whereit supports its share of the plane's weight. The piston areas are sizedto achieve the needed proportions between main gear 305 squat, and nosegear 310 stroke-out to pivot about the C_(G). These pistons mate toconventional oleo struts 315 with the wheels, tires, and brakes.

The systems 300, 400 can function for takeoff rotation and landingde-rotation. During other phases like taxi, takeoff, roll up to V₁, orlanding after de-rotation, the plane can be locked in the level, orground, position. Under braking the systems 300, 400 can be locked toprevent the braking force from jacking the nose upward. This can be doneby closing the hydraulic valve 410 in the hydraulic pipe 405 between thenose gear 310 and main gear 305. Preventing flow from the nose gear 310to the main gear 305 hydraulically locks the system. A slight bias innose gear 310 hydraulic or lever ratio can make the plane very slightlynose-heavy. This can prevent the plane from pitching upward if thehydraulic valve 410 is opened during ground operations.

As discussed above, the hydraulic valve 410 can be opened whenever thepilot shows intent to change pitch attitude, but should not open if thestick is bumped or slightly nudged. Thus, a stick-force dead-band ofapproximately ±5 lbs. can be used. Since takeoff rotation is a safetycritical function, the systems 300, 400 can be a fail-open system.

The hydraulic pipe 405 (e.g., piping, a hose, or tubing) from the maingear 305 to the nose gear 310 causes the plane to pivot about the C_(G)without jacking the C_(G) vertically. This results in dramaticallysmaller control moments required to place the plane at lift-off attitudefor takeoff. The small moment means a small elevon 110 down-load doesn'toppose the plane's natural coefficient of lift at minimum unstick speed,or C_(LVMU), capability. This improvement in the down force required atV_(MU) results in approximately 29% better C_(LVMU) at a fixedGround-Angle-Limit (GAL) which is equivalent to about 25 knots inminimum unstick speed (V_(MU)) benefit.

In one example, the nose gear 310 can be located about three times asfar from the C_(G) in the forward direction, as the main gear 305 is inthe aft direction. In this configuration, all three landing gears bearapproximately the same “maximum” vertical and braking loads. The maingear 305 maximum load is the static load and it is approximately thesame as the maximum nose gear 310 load which occurs during braking. Thismeans that all three landing gears can use common parts except for theaddition of steering to the nose gear 310. The 3-to-1 leveragedifference means that for every foot the main gear 305 squats atrotation, the nose gear 310 extends 3 times as much. This is exactlywhat is needed for the plane's virtual pivot point to be near the C_(G).

A normal blended wing aircraft with a braking coefficient of 0.7 on themain-gear causes the nose-gear to bear approximately 23% of the totalairplane weight (so-called “weight transfer”) versus a typical load ofaround 3%-5%. Since the nose gear 310 has no brakes, however, theeffective airplane braking coefficient is only about 0.55. With thesystems 300, 400 disclosed herein, however, the nose gear 310 is moreheavily loaded, so brakes may be added while preserving the neededsteering power.

Weight transfer under braking can now be exploited since the nose gear310 has brakes. The airplane braking coefficient can now beapproximately the same as the individual braking coefficient. That is,the full braking effect is preserved for an approximately 29%improvement in overall braking performance. This also benefits takeofffield length and landing field length. Brakes on the nose gear 310 donot diminish steering relative to a traditional lightly loaded nose gear310 because the high nose gear 310 load reduces the steer angle requiredto generate a side-load on the nose gear 310. The amount of the frictiondevoted to steering can be very small, allowing full braking whilesteering, similar to in a car.

The systems 300, 400 add hardware that is absent in traditional landinggear. This can increase weight and cost. Landing gear weight includeswheels, tires, brakes, and struts. The struts generally account forabout 35% of the total gear weight. Adding the systems 300, 400 roughlytriples the strut weight component. Therefore we can expect the systems300, 400 to increase gear weight by 70%. For most airplanes the totallanding gear complement is approximately 4% of gross weight (GW) so thesystems 300, 400 hardware adds approximately 2.8% to the GW.

The standard wheel wells for a blended wing aircraft, however, aregenerally within the pressure vessel. Thus, the pressure vessel surfacearea is increased by the side-walls of the wheel well addingapproximately 12% to the wall area. To compensate for the wheel wellpenetrations, the payload compartment is required to grow another 15%.Thus, the pressure bearing wall area of the pressure vessel increasesapproximately 30%. The pressure vessel of a typical blended wingaircraft accounts for 9% of the total GW. As a result, the price of thegear penetrations is 2.7% in GW. This almost entirely cancels the costof the systems 300, 400. This also does not take into account thereduced drag of the smaller pressure vessel, which more than offsets theweight of the systems 300, 400.

The Federal Aviation Regulations set forth requirements for determiningtakeoff and landing field lengths. Landing is relativelystraight-forward because there is an air distance between the point atwhich the plane is 50 ft above-ground-level and the touchdown point.Next there is a distance where the plane is de-rotated so all 3 landinggears are on the ground. Finally there is a braked deceleration to zerospeed. The systems 300, 400 reduce the braked deceleration portion byapproximately 500 ft., equivalent to an approximately 11% reduction inlanding field length (LFL).

CONCLUSION

System and methods for mechanically rotating an aircraft about its C_(G)are disclosed. The system can use a variety of actuators to lower themain landing gear and/or raise the nose landing gear to achieve adesired AOA for takeoff, landing, or other flight or ground regimes. Thesystem can use a passive hydraulic system, an active hydraulic system,or electrical or pneumatic actuators, among other things. The system canenable the aircraft to rotate about the C_(G), to reduce the aerodynamiccontrol forces required due to weights and balances, landing gear orwing placement, or other factors. The system can be used on blendedwing, delta wing, tube and wing, and other aircraft configurations.

While several possible embodiments are disclosed above, embodiments ofthe present invention are not so limited. For instance, while severalpossible configurations of hydraulic cylinders, linear actuators,valves, and motors, other suitable actuators and controls could beselected without departing from the spirit of embodiments of theinvention. In addition, the location and configuration used for variousfeatures of embodiments of the present disclosure can be variedaccording to a particular aircraft, airport, or landing gear design thatrequires a slight variation due to, for example, size or weightconstraints, runway length, aircraft type, or other factors. Suchchanges are intended to be embraced within the scope of the invention.

The specific configurations, choice of materials, and the size and shapeof various elements can be varied according to particular designspecifications or constraints requiring a device, system, or methodconstructed according to the principles of the invention. Such changesare intended to be embraced within the scope of the invention. Thepresently disclosed embodiments, therefore, are considered in allrespects to be illustrative and not restrictive. The scope of theinvention is indicated by the appended claims, rather than the foregoingdescription, and all changes that come within the meaning and range ofequivalents thereof are intended to be embraced therein.

What is claimed is:
 1. A landing gear system for an aircraft comprising: a nose gear disposed proximate a front of the aircraft, the nose gear controllably movable between a first position, in which the nose gear is retracted, and a second position, in which the nose gear is extended; a main gear, hydraulically or pneumatically coupled to the nose gear, the main gear disposed proximate a rear of the aircraft, the main gear controllably movable between a third position, in which the main gear is extended, and a fourth position, in which the main gear is retracted; wherein, in a ground position, the nose gear is in the first position and main gear is in the third position and a fuselage of the aircraft is substantially level with the ground; wherein, in an angle-of-attack (AOA) position, the nose gear is in the second position and the main gear is in the fourth position and the fuselage of the aircraft is rotated to positive AOA with respect to the ground; wherein retracting the nose gear causes the main gear to extend; and wherein extending the nose gear causes the main gear to retract.
 2. The system of claim 1, wherein: the AOA position comprises a takeoff AOA position; and the landing gear system rotates the fuselage of the aircraft to a predetermined takeoff AOA (α_(TO)).
 3. The system of claim 1, wherein: the AOA position comprises a landing AOA position; and the landing gear system rotates the fuselage of the aircraft to a predetermined landing AOA (α_(L)).
 4. The system of claim 1, further comprising: a main cylinder coupled to the main gear; a nose cylinder coupled to the nose gear; and at least a valve coupled to the main cylinder and the nose cylinder, the at least a valve comprising a closed position and an open position; wherein, in the closed position, the at least a valve prevents a flow of a fluid between the nose cylinder and the main cylinder; wherein, in the open position, the at least a valve enables the flow of the fluid between the nose cylinder and the main cylinder; and wherein the flow of fluid between the nose cylinder and the main cylinder causes the nose cylinder to raise and the main cylinder to lower, or vice-versa.
 5. The system of claim 4, wherein: the at least a valve comprises at least a hydraulic valve; and the fluid comprises hydraulic fluid.
 6. The system of claim 4, wherein: the at least a valve comprises at least a pneumatic valve; and the fluid comprises one of air or nitrogen.
 7. A rotating landing gear system for an aircraft, comprising: a nose gear disposed proximate a front of the aircraft; two or more main gears disposed proximate a rear of the aircraft; a nose cylinder mechanically coupled to the nose gear to move the nose gear between a first position, in which the nose gear is retracted, and a second position, in which the nose gear is extended; two or more main cylinders, each mechanically coupled to the two or more main gears to move the main gears between a third position, in which the main gears are extended, and a fourth position, in which the main gears are retracted; and at least a hydraulic valve hydraulically coupling the nose cylinder and the main cylinders to enable fluid to flow between the nose cylinder and the main cylinders to change an angle-of-attack (AOA) of the aircraft with respect to the ground.
 8. The system of claim 7, wherein the nose cylinder and the two or more main cylinders are sized and shaped such that, when the at least a hydraulic valve is opened with the aircraft on the ground, the nose cylinder and the two or more main cylinders maintain their relative positions.
 9. The system of claim 8, wherein a total piston surface area of the two or more main cylinders is greater than a total piston surface area of the nose cylinder; and wherein a distance between the nose gear and a center-of-gravity (CG) of the aircraft is greater than a distance between the two or more main gears and the CG.
 10. The system of claim 7, wherein the nose cylinder and the two or more main cylinders are sized and shaped such that, when the at least a hydraulic valve is opened with the aircraft in the air, the nose cylinder retracts and the two or more main cylinders extend.
 11. The system of claim 10, wherein a total weight of the two or more main gears is greater than a total weight of the nose gear.
 12. The system of claim 7, further comprising: a first set of two or more levers, each disposed between the two or more main gears and the two or more main cylinders; a second lever disposed between the nose gear and the nose cylinder; and wherein the first set of levers and the second lever are sized and shaped such that a first movement of the two or more mains gears causes a proportionately greater second movement of the nose gear.
 13. The system of claim 12, wherein the second movement is three times greater than the first movement.
 14. A method for mechanically rotating an aircraft to a desired angle-of-attack (AOA) during a procedure, the method comprising: determining that the aircraft has reached a predetermined speed; opening at least a hydraulic valve hydraulically coupled to a nose gear cylinder and a main gear cylinder to enable fluid to flow from the nose gear cylinder to the main gear cylinder, or vice-versa, to achieve the desired AOA; determining that a nose gear and a main gear have moved to a first position in which the aircraft is positioned at the desired AOA; and closing the at least a hydraulic valve to hydraulically lock a position of the nose gear relative to the main gear.
 15. The method of claim 14, wherein: the procedure comprises a takeoff; and the predetermined speed comprises a rotation speed (V₁) for the aircraft.
 16. The method of claim 14, wherein: the procedure comprises a landing; and the predetermined speed comprises a reference landing speed (V_(REF)) for the aircraft.
 17. The method of claim 14, further comprising: determining that a predetermined force has been applied to a control stick of the aircraft that is greater than or equal to a minimum stick force prior to opening the at least a hydraulic valve.
 18. The method of claim 14, wherein: the first position comprises a ground position in which the nose gear and the main gear place the aircraft at substantially zero AOA; and wherein the predetermined speed comprises a taxiing speed.
 19. The method of claim 14 further comprising: activating a hydraulic pump to cause the fluid to flow from the nose gear cylinder to the main gear cylinder, or vice-versa, to achieve the desired AOA.
 20. The method of claim 14, further comprising: determining that a force on at least one of the nose gear or the main gear is greater than or equal to a minimum force indicating that the aircraft is on the ground; opening the at least a hydraulic valve to enable fluid to flow from the nose gear cylinder to the main gear cylinder, or vice-versa; determining that the nose gear and the main gear have moved to a ground position in which a fuselage of the aircraft is substantially level with the ground; and closing the at least a hydraulic valve to hydraulically lock the position of the nose gear relative to the main gear. 